1. Field of the Invention
The present invention relates to a tire air pressure detecting device for monitoring whether a tire air pressure is lowered when a main switch is turned on after a motor vehicle has stopped with the main switch being turned off, and reducing the consumption of a battery on transmission side.
2. Description of Background Art
One tire air pressure detecting device for detecting the air pressure of a tire and dealing with a situation where the air pressure of the tire suffers an abnormal condition is known, for example, from Japanese patent laid-open No. Hei 9-240446 entitled “Brake Control Device.”
As shown in FIG. 4 of the above publication, the disclosed tire air pressure detecting device has a left front wheel air pressure sensor 14 (the reference numerals are from the description of the publication) and a right front wheel air pressure sensor 15 for directly measuring the air pressures of tires. A receiver (no reference numeral) is provided for receiving signals representing the air pressures which are transmitted from the left and right front wheel air pressure sensors 14, 15. A controller 30 receives the signals representing the air pressures from the receiver. A warning unit 8 is provided for issuing a warning based on a signal from the controller 30 if the air pressures of the tires become lower than a predetermined value.
With the arrangement disclosed in the above publication, while the motor vehicle is stopping with a main switch being turned off, since the receiver and the controller 30 on the vehicle body are not energized, the receiver is unable to receive signals of tire air pressure when the air pressures of the tires drop while the motor vehicle is stopped.
When the main switch is then turned on and the receiver and the controller 30 are operated, unless latest air pressure signals are transmitted from the left and right front wheel air pressure sensors 14, 15 until the motor vehicle starts to move after the main switch is turned on, the warning unit 8 is not activated and the driver is unable to know the tire air pressure drop when the motor vehicle starts to move.
One solution would be to construct the left and right front wheel air pressure sensors 14, 15 to transmit signals at all times. However, since the left and right front wheel air pressure sensors 14, 15 and a transmitter associated therewith would be required to be energized at all times, the consumption of the battery which energizes the left and right front wheel air pressure sensors 14, 15 and the transmitter would be accelerated.